Composite materials are more and more frequently used in many industrial applications, such as in aeronautical, automobile and rail transport applications. The particular structure of these materials enables them to embody pieces with extremely varied shapes and provide these pieces with the desired characteristics by means of a suitable orientation of the fibers they contain.
The method of the invention is in particular applicable to the production of aeronautical structures made of a composite material, these structures being extremely delicate to embody. These structures are pieces with strong curves having non-extractable shapes, such as aircraft fuselage frames, door framing stiffeners, support reinforcements disposed at the edges of holes, etc.
In order to embody such pieces, one first solution consists of producing a rectangular preform plate by draping edge to edge and in successive sheets strips of unidirectional fibers or fibrous fabrics impregnated with resin. Then a preform is cut from this plate, this preform having the shape of the piece to be embodied. So as to give this preform the section of the piece, it is subjected to one or several forming operations during which the uneven sections of the piece are shaped with a aid of a suitable tool.
This known method has a large number of drawbacks. Accordingly, the operation consists of cutting a preform having the shape of the piece to be embodied from a large preform plate resulting in a high discard rate, which increases the dimensions of the piece.
In addition, this method involves embodying a piece in which the distribution of the fibers is totally anisotropic. This in particular complicates the task of the engineering office which needs to take into account when designing the piece the evolution of the mechanical characteristics arising from this distribution of the fibers. Moreover, the unhomogeneous distribution of the fibers in the preform strip, as well as the carrying out of forming operations on a plain strip, result in area contractions and elongations unable to be accepted by the fibrous products in certain zones, which results in the formation of folds, etc. Finally, this method is a long method and cumbersome to implement.
In one variant of this known production method, instead of embodying by draping a rectangular preform plate and then of cutting a preform from this plate, a preform is produced directly by draping whose shape approximates the contour of the piece to be embodied.
If this technique makes it possible to considerably reduce the discard rate, it significantly increases the draping time and does not resolve any of the aforementioned drawbacks.
In the document FR-A-88 10984, another method is described making it possible to embody a flat preform having the shape of the piece desired to be obtained from a rectilinear preform strip cut from a larger flat preform plate. This method consists of curving inwards the preform strip inside its plane, for example with the aid of a conical roller machine. Thus, a piece is obtained in which the distribution of the fibers is homogeneous over its entire length with a discard rate of virtually nil.
However, so as to provide this piece with its definitive section, the latter needs to be subsequently subjected to one or more forming operations. During these operations, the sections internal to the radius of curvature of the piece are shaped by the apparent elongation of the evolute of the fibers which are then stretched. On the other hand, the sections outside the radius of curvature and previously deformed with the aid of the conical roller machine are shaped by the apparent area contraction of the evolute of the fibers outside the radius of curvature, which are then compressed.
In pieces produced in this way, these compressed fibers singe owing to the friction stresses existing between them and the reduction of the evolute is absorbed by the undulation of the fibers. The undulated fibers then lose all their stiffness and resistance characteristics in the stratified piece, which then more strongly stresses the binding resin. This results in a falling off of the characteristics, this proving to considerably damage structure pieces.
In addition, this method requires, like the preceding methods, a relatively large number of stages, thus increasing the duration and cost of the method.